The present invention relates to methods and apparatus for reclaiming or recovering glycols, and more particularly relates, in one non-limiting embodiment, to methods and apparatus for reclaiming or recovering monoethylene glycol by removing salts therefrom.
For several decades monoethylene glycol (MEG) and methanol have been the primary chemicals injected offshore to inhibit the formation of hydrates in oil and gas production pipelines and related facilities. At fields that require persistent inhibition, the cost of replacing hydrate inhibitor chemical that is lost to the gas and hydrocarbon liquid product streams is a determining factor in selecting the inhibitor. Methanol solubility in gas and liquid hydrocarbon product streams may be two or more orders of magnitude higher than MEG solubility. This creates a strong economic incentive to use MEG despite the greater quantity of MEG needed per degree of hydrate temperature suppression. Further, methanol is an unwanted contaminant in hydrocarbon sales products.
Nevertheless, the adoption of MEG over methanol has taken some time to occur, due in part to familiarity with methanol and perceptions in operating difficulties in recovering and recycling MEG. There have been concerns about detrimental effects that saline formation water had on conventional MEG reconcentrator units. Salts are non-volatile, and they will remain in the lean MEG during regeneration as the water is boiled off. If salt enters the MEG system with production fluids, either continuously or in periodic slugs, its concentration in the system will increase during each regeneration cycle until the solubility limit is finally reached and precipitation occurs. Unless MEG is reclaimed by removing the salt, serious fouling and plugging of equipment and flowlines may result. Depending on the magnitude of the contamination problem, MEG reclamation may be accomplished by either continuous or intermittent (i.e. batch) removal from either the total circulation stream or else a partial stream.
Many MEG reclamation projects involve the removal or reduction of alkaline earth metal salts such as calcium chloride (CaCl2) from the contaminated MEG stream. In some reclamation efforts, the salts are converted to carbonates which may be removed as precipitated particulates. Current or conventional removal methods used include centrifuges, diatomaceous earth (DE) filtration and separation tanks with relatively long retention times (e.g. on the order of 10 hours).
It would be desirable if methods and apparatus were devised that could easily remove salts and/or carbonates from MEG streams with greater efficiency than at present.